1. Field of the Invention
The present invention relates to a current sensor having a magnetic circuit with a gap and a measuring device able to supply a voltage representative of a magnetic field Hlex produced in the gap by a current, when the latter passes through the magnetic circuit and whose value is a function of the intensity of said current.
The intensity of the current can be constant in time or can be an alternating current, whose frequency does not exceed approximately 10.sup.5 Hz. Moreover, the current can be transported by an electrical conductor or can be the current of a beam of electrically charged particles.
The invention more particularly applies to the measurement of low electrical intensities, e.g. a few dozen mA.
2. Discussion of the Background
Documents (1) "The grip current tester E1", Chauvin Arnoux, Electronique Radio Plans 520, pp.19 to 21, 1991 and (2) Electro PJP, "Technical report of the designer", pp. 1 to 9 disclose current sensors having a magnetic circuit with a gap, together with a magnetic sensor constituted by a Hall effect sensor or probe placed in the gap. When an electric current passes through the magnetic circuit, a magnetic field is produced in the gap and said field is proportional to the intensity of the current and inversely proportional to the length of the gap.
In connection with Hall effect sensors reference can be made to document (3) "Hall effect devices", R.S. Popovic, Sensors and Actuators, 17,1989, pp. 39 to 53.
Other sensors using the Hall effect are known from documents (4) FR-A-2,584,195 (Chauvin-Arnoux) and (5) EP-A-194,225 (LEM SA).
Current sensors using the Hall effect suffer from disadvantages and in particular a low resolution of approximately 1 microtesla, an offset during a measurement of approximately 1 mT, they have a noise which is inversely proportional to the frequency of the measured signal and they are sensitive to mechanical stresses (cf. in particular document (3), p.43).
All these parameters, which are linked with a temperature sensitivity of approximately 100 ppm/.degree. C. have the effect of limiting the current resolution of the sensors using the Hall effect to a value of at the best 1 mA.
Other current sensors known from documents (6) U.S. Pat. No. 4,616,174 (Jorgensen) and (7) "A new current sensor based on the measurement of the apparent coercive field strength", Derac Son and Johannes D. Sievert, IEEE transactions on instrumentation and measurement, Vol. 38, No. 6, Dec. 1989, pp. 1080 to 1082 make use of the non-linearities of the magnetic material constituting the aforementioned magnetic circuit. In the devices described in (6) and (7), the magnetic circuit is exposed to a pulsed magnetic field, whose mean value is zero.
The existence of a current to be measured has the effect of bringing about the appearance of paired components for the magnetic flux present in the magnetic circuit. It is therefore possible to gain approximately one order of magnitude for the current resolution compared with Hall effect sensors, but this involves a certain complication of the electronic circuits used.
In this connection reference can also be made to document (8) Documentation of Danfysik Jyllinge-Danmark on the NTRASTAB 860R (registered trademark) current transducer.